The present invention relates generally to earth moving machinery and more particularly to hydraulic transmission systems for tracked vehicles of the type comprising:
two separate hydrostatic transmissions for driving the right hand and left hand tracks of the vehicle, each including a variable-displacement hydraulic pump provided with hydraulically-operated control means for varying the pump displacement, PA1 a hydraulic pilot pump, PA1 a lever control device, for controlling the speed of the vehicle, said device being interposed between the pilot pump and the said control means for the pumps of the two hydrostatic transmissions and providing at its output, for each position of its control lever corresponding to a predetermined speed of the vehicle, PA1 a constant pressure which is less than the output pressure of the pilot pump. PA1 a first input communicating with the output of the speed control device, PA1 a second input communicating with the output of the pilot pump, PA1 a first output communicating with the control means associated with the pump of one of the two hydrostatic transmissions, PA1 a further output communicating with a discharge tank, PA1 a duct connecting the first input to the first output and including a constriction, PA1 a main chamber communicating with the duct downstream of the constriction, PA1 a slide valve member having a first operative end position in which the main chamber communicates with the second input, a second operative end position in which the main chamber communicates with the further output, and a central neutral position, PA1 two control members arranged to act on respective ones of the two ends of the slide valve member to urge it towards one or other of its two operative end positions, PA1 two springs interposed between respective ones of the control members and the corresponding end of the slide valve member, PA1 first and second end chambers adjacent respective ones of the two ends of the slide valve member, the first end chamber communicating with the main chamber, the second end chamber communicating with the first input. PA1 a first input communicating with the forward-movement output of the speed control device, PA1 a second input communicating with the output of the pilot pump, PA1 a third input communicating with the rearward-movement output of the speed control device, PA1 a first output communicating with the forward-movement input of the control means for the pump of one of the two hydrostatic transmissions, PA1 a second output communicating with the rearward-movement input of the control means of the pump of the other hydrostatic transmission, PA1 a third output communicating with the discharge tank, PA1 a first duct connecting the first input to the first output and including a constriction, PA1 a second duct connecting the third input to the second output, and also including a constriction, PA1 a main chamber communicating with the first and second ducts downstream of their respective constrictions, by means of a selector valve, PA1 a slide valve member having a first operative end position, in which the main chamber communicates with the second input, a second operative end position in which the main chamber communicates with the third output, and a neutral central position, PA1 two control members arranged to act on respective ones of the two ends of the slide valve member to urge it towards one or other of its operative end positions, PA1 two springs interposed between respective ones of the control members and the corresponding end of the slide valve member, PA1 first and second end chambers adjacent respective ones of the two ends of the slide valve member, the first end chamber communicating with the main chamber, the second end chamber communicating with the first input and the third input through a further selector valve.
In the operation of construction machinery vehicles, tracked vehicles are utilized having a transmission system of the type specified above. Although, on the one hand these vehicles have the particular merits of ease of handling and maneuverability by virtue of the fact that the two tracks are driven by two hydrostatic transmissions that are separate from each other; on the other hand, they have the disadvantage of being subject to deviations from rectilinear movement because of the inevitable differences that exist between the characteristic parameters of the two hydrostatic transmissions (volumetric efficiency of the hydraulic pumps or the hydraulic motors of the two transmissions, characteristics of the two control units) due to manufacturing inaccuracies.
Illustrations depicting the manner in which various solutions have been attempted to solve existing problems may be found in U.S. Pat. Nos. 3,765,173; 3,812,925; 3,862,668; 3,908,374; 4,019,596 and 4,175,628.